The present invention relates to highly oriented conducting layers on SiO2/Si and glass. This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
Conductive electrodes such as ruthenium oxide (RuO2) and lanthanum strontium cobalt oxide (La0.5Sr0.5CoO3 generally referred to as LSCO) have been extensively studied as electrode materials for thin film capacitors in which ferroelectric/paraelectric materials are used as dielectrics. Exemplary structures are shown in the following patents. U.S. Pat. No. 5,519,235 relates to a ferroelectric capacitor heterostructure wherein an amorphous SiO2 surface on a silicon wafer surface is initially primed or coated with a thin layer of titanium, tantalum or titanium dioxide and then coated with a thin layer of a metal such as platinum for the subsequent polycrystalline growth of metallic oxide electrode materials such as LSCO, RuOx, and SrRuO3. U.S. Pat. No. 5,270,298 relates to formation of crystalline metal oxide thin films such as LSCO employing a layered perovskite, such as bismuth titanate (BTO), template layer to initiate c-axis orientation in LSCO and PZT overlayers. Similarly, U.S. Pat. No. 5,248,564 used a layer of BTO on a silicon dioxide layer prior to a layer of LSCO. The interlayers of BTO are c-axis oriented only, i.e., uniaxially oriented.
Conductive RuO2, which has a rutile structure and tetragonal unit cell with a=b=0.44902 nanometers (nm), c=0.31059 nm, has been widely studied recently due to its unique properties compared to other oxide materials. High electrical conductivity, thermal stability, and chemical resistance make RuO2 very attractive in a variety of applications. Amorphous or polycrystalline RuO2 thin films have been deposited on a variety of substrates, such as oxidized silicon (SiO2/Si), silicon (Si), quartz, glass, and magnesium oxide (MgO). Recently, epitaxial RuO2 thin films have been grown on lattice matched substrates such as LaAlO3, yttria-stabilized zirconia (YSZ), YSZISi, and sapphire. The growth of highly textured RuO2 on SiO2/Si is more relevant in electronic devices since SiO2 is almost exclusively used as a field oxide, as a passivation layer, or as an isolation material in silicon-based circuitry. Highly textured RuO2 is preferable for use as electrodes in dielectric thin film capacitors because well oriented electrodes can further enhance the electrical and dielectric properties of dielectric materials. Nevertheless, all the previous RuO2 films deposited on SiO2/Si show polycrystalline or uniaxial normal alignment (random in-plane orientation).
The conductive oxide La0.5Sr0.5CoO3 (LSCO), which has a psuedo-cubic lattice constant of 0.3835 nm and a room temperature resistivity of 90 xcexcxcexa9-cm, has been extensively studied as an electrode material for ferroelectric thin film capacitors, where the dielectric materials can be PbZrxTi1xe2x88x92xO3 (PZT) or lanthanum-modified PZT. The improved device performance obtained by using LSCO as an electrode material, compared with the use of conventional platinum, has been attributed to the better structural/chemical compatibility and the cleaner interface (less charged defects) between LSCO and the dielectric materials. Fewer oxygen vacancies within the near interface region of the ferroelectric layer may also contribute to superior device performance.
For applications of LSCO films such as electrodes for nonvolatile ferroelectric random access memories (NFRAMs), epitaxial and/or well-textured LSCO films are preferable. The reduced grain-boundary scattering from an epitaxial LSCO film leads to lower resistivity of the film, which is a prerequisite for high frequency applications. As a bottom electrode and/or seed layer for ferroelectric thin film capacitors, well textured LSCO films also induce epitaxial or preferential oriented growth in subsequently deposited ferroelectric films. This is important since a highly oriented ferroelectric layer can produce a larger remnant polarization compared to a randomly oriented ferroelectric layer.
Epitaxial and/or well-textured LSCO films have been grown on SrTiO3, MgO, LaAlO3 and yttria-stabilized zirconia (YSZ). The growth of well-textured LSCO on technically important SiO2/Si is more relevant in microelectronic devices since SiO2 is almost exclusively used as a field oxide, a passivation layer, and/or an isolation material in silicon-based circuitry. Highly oriented LSCO on SiO2/Si has been accomplished by using Bi4Ti3O12 as a template (see U.S. Pat. No. 5,248,564). Nevertheless, the LSCO films deposited on SiO2/Si by this method show only uniaxial normal alignment with random in-plane orientation. The growth of well-textured or biaxially oriented LSCO films (both normal to and in the film plane) on SiO2/Si has not previously been accomplished.
Improved electric/dielectric properties of PbZrxTi1xe2x88x92xO3 (PZT), BaTiO3, SrTiO3, and BaxSr1xe2x88x92xTiO3 have been observed by using LSCO and/or RuO2 as the electrode materials, as compared to the use of conventional platinum electrodes. Both amorphous and/or polycrystalline thin films of RuO2 or LSCO have been previously deposited on SiO2/Si substrates. The growth of highly oriented or well textured RuO2 and LSCO thin films on SiO2/Si substrates is preferable for electrodes in dielectric thin film capacitors as such highly oriented or well textured electrodes can further enhance the electrical and dielectric properties of subsequently deposited dielectric materials.
Existing technology does not provide highly oriented conductive oxides on substrates such as amorphous SiO2/Si and glass. The difficulties of forming such oriented layers on amorphous or polycrystalline substrates are due to seed growth. A structure of epitaxial RuO2/YSZ/SiO2/Si (with a SiO2 layer greater than 100 nm in thickness) has been previously achieved by additional high temperature processing steps (see U.S. Pat. No. 5,912,068 by Jia for xe2x80x9cEpitaxial Oxides on Amorphous SiO2 on Single Crystal Siliconxe2x80x9d). Such high processing temperatures (greater than 900xc2x0 C.) present serious problems for processing on silicon and glass. For example, such a high temperature process cannot be used where there are active devices located on the silicon or where the melting temperature of the substrate is lower than the processing temperature.
It is an object of the present invention to provide a method of forming highly oriented conductive oxides on SiO2/Si substrates, such highly oriented conductive oxides preferably characterized as biaxially oriented.
Another object of the present invention is to provide a low temperature method of forming highly oriented conductive oxides on SiO2/Si substrates.
Another object of the invention is to provide a thin film structure including a thin layer of biaxially oriented YSZ on a SiO2/Si substrate for subsequent deposition of highly oriented conductive oxides, said thin layer of oriented YSZ formed by ion-beam-assisted-deposition (IBAD).
Still another object of the present invention is to provide a thin film structure including a structure of an oriented layer of Ba0.5Sr0.5TiO3 (BSTO) and/or Ba1xe2x88x92xSrxTiO3 0xe2x89xa6xc3x97xe2x89xa61) on a biaxially oriented layer of RuO2 on an ion-beam-assisted-deposited (IBAD) layer of YSZ on a SiO2/Si substrate.
Yet another object of the present invention is to provide a thin film structure including a structure of a biaxially oriented layer of La0.5Sr0.5CoO3 on a biaxially oriented layer of CeO2 on an ion-beam-assisted-deposited layer of YSZ on a SiO2/Si substrate.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention can be summarized as a thin film structure including a silicon substrate having a layer of silicon dioxide on a surface thereof, said silicon dioxide layer having a thickness of at least about 100 nanometers, and a layer of cubic oxide material deposited upon the layer of silicon dioxide by ion-beam-assisted-deposition, said layer of cubic oxide material characterized as biaxially oriented. Preferably, the cubic oxide material is yttria-stabilized zirconia.
The present invention can further include additional thin layers of biaxially oriented ruthenium oxide or lanthanum strontium cobalt oxide upon the layer of yttria-stabilized zirconia. In the case of lanthanum strontium cobalt oxide, an intermediate layer of cerium oxide is situated between the yttria-stabilized zirconia layer and the lanthanum strontium cobalt oxide layer. Also, the present invention can further include a layer of barium strontium titanium oxide upon the layer of biaxially oriented ruthenium oxide or lanthanum strontium cobalt oxide.
In addition to the thin film structures of the present invention, the present invention includes methods of forming such thin film structures, such a method including a low temperature deposition of a layer of a biaxially oriented cubic oxide material upon the silicon dioxide surface of a silicon dioxide/silicon substrate.